A compositional heterojunction structure occurs when a semiconductor material having a given bandgap is grown onto a semiconductor material having a different bandgap. If the conduction band minimum and valence band maximum in the narrower bandgap material are aligned or nested within the bandgap of the other material, the heterojunction is Type I. If, however, the conduction band minimum and valance band maximum are staggered the heterojunction is Type II.
Because of a relatively direct transition, the radiative recombination process in Type I or nested heterojunctions is generally more efficient than in Type II heterojunctions. Thus, Type I heterojunctions are preferable in semiconductor devices which are utilized as light sources in optical communication systems. Further, in optical communication systems, the wavelength at which the optical source emits radiation is important, as most optical fibers used in long-distance links have an attenuation minimum at approximately 1.55 .mu.m. For this reason III-V alloys tailored to emit at this wavelength are frequently selected. To achieve this wavelength an alloy based on the InGaAsP/InP system is commonly adopted. This alloy structure is known to yield Type I heterojunctions which from an efficiency standpoint is advantageous. There is, however, an inherent characteristic of Type I heterojunctions which adversely affects efficient operation of lasing devices based on such junctions. This characteristic is the tendency for the threshold current of the laser to increase with temperature, the threshold current density increasing approximately exponentially with temperature. This temperature sensitivity, apart from other limitations on device performance, causes temperature induced chirp or wavelength shift. This wavelength shift in conjunction with the inherent dispersion in an optical fiber limits the bandwidth and/or transmission distance that may be obtained in a long-haul optical transmission system.
As will be discussed in greater detail herein, a Type II heterojunction, appropriately situated with respect to the laser active region, can induce a recombination current which decreases approximately exponentially in relation to increasing temperature. Thus, the combination of a Type I and Type II heterojunction in a semiconductor laser is presented herein as a means to reduce the temperature sensitivity of the operational characteristics.